Technically and officially, only members of Congress can actually introduce new laws. Any member of either house of Congress can propose a new law. Only members of the House of Representatives can introduce laws having to do with "raising revenue."

If you are asking this question more broadly, new laws are often proposed by the President or by interest groups as well as by members of Congress. Presidents and interest groups have agendas that they want to move forward, and a major way to do this is by proposing legislation. Of course, they still have to get a member of Congress to actually introduce the bill.

Overall, then, the actual origin of many bills is with presidents and interest groups. However, only members of Congress may actually introduce the bills.